The present invention relates to a data recording/reproducing apparatus and a data recording/reproducing method for use with a randomly accessible recording medium, as well as a computer program and a recording medium for data recording and reproduction. More particularly, the present invention relates to techniques for causing a magnetic head to scan a magnetic disc medium such as a hard disc for writing and reading data to and from the medium, with emphasis on reducing the time of access to desired data storage locations on the medium during a stable data recording/reproducing operation.
Along with technological progress in information processing, data communication and other related fields has come a growing need for reusing information that was prepared and edited in the past. Information storage technology is thus becoming more important than ever. Varieties of information recording apparatuses using diverse media including magnetic tapes and magnetic discs have been developed and have come into general use today.
Of these apparatuses, the hard disc drive (HDD) is an auxiliary storage apparatus utilizing the magnetic recording method. An HDD unit incorporates a number of magnetic discs which constitute recording media and which are rotated at high speed by a motor. A coat of magnetic material such as iron oxide and chrome-cobalt alloy is plated or laminated over the media. In operation, the magnetic head is moved radially over the rotating media to scan their surface in order to magnetize the media suitably for writing or reading data to or from the media.
Hard disc drives have gained widespread use. Illustratively, the HDD is used as a standard external storage device for personal computers. The HDD stores the operating system (OS) necessary for starting up the computer, has various application programs installed thereon, and retains files that have been created and edited. Normally, the HDD is connected to the computer through a standardized interface such as IDE (Integrated Drive Electronics) or SCSI (Small Computer System Interface); the storage space of the HDD is managed by a file system, which is a subsystem of the operating system such as FAT (File Allocation Table).
Today, the hard disc drive is getting larger than ever in capacity. The enhanced capacity is enabling the HDD to serve not only as a conventional auxiliary storage device for computers but also as a hard disc recorder for accumulating audio-visual (AD) content that has been broadcast and received. This and more new applications are envisaged for the HDD. The apparatus is thus beginning to be used for recording diverse kinds of content.
Consideration will now be given to how the hard disc is physically formatted and how data are written and read to and from the hard disc where the hard disc drive is used as an auxiliary storage device for computers.
A large number of “tracks” are formed concentrically on the hard disc as segments to which to record data. The tracks ranging from the radially outermost to the innermost zones of the disc are numbered in ascending order starting with 0. The larger the number of tracks formed on the disc surface, the larger the storage capacity of the medium.
Each of the tracks is divided into “sectors” each serving as a recording unit. Data are written to and read from the disc usually in increments of sectors. While the sector size varies from one medium to another, each sector on the hard disc is generally fixed to a size of 512 bytes. In view of the efficiency in using the media, the number of sectors per track is made larger the closer the track is to the outermost zone of the disc because each track has a larger circumference than its immediately adjacent inner track. This scheme is called zone bit recording.
Where the zone bit recording scheme is adopted, the recording density of each of the tracks is averaged. One disadvantage of this scheme is that the data transfer rate turns out uneven from one track to another. The data transfer rate gets lower over tracks that are located closer to the innermost zone of the disc.
In the case of a hard disc drive in which a plurality of media are stacked concentrically, the tracks with the same track number on the media may be regarded as located cylindrically. In that sense, these tracks are collectively called a cylinder. Each cylinder is assigned the same number as that of the tracks constituting it. The cylinders ranging from the radially outermost to the innermost zones of the disc are numbered in ascending order starting with 0. A plurality of magnetic heads located between the stacked media in inserted fashion are moved collectively between cylinders.
So-called CHS mode is one way of having a target sector addressed. This is a method for gaining access to desired data by addressing PBA (physical block address) in terms of C (cylinder), H (head) and S (sector), in that order.
Under the CHS scheme, the computer acting as the host to the hard disc drive has a limit on addressable CHS parameters. The hard disc drive getting larger in capacity can no longer be dealt with by the CHS mode for lack of addressing capacity. In order to bypass this bottleneck, what is known as LBA (logical block address) mode is adopted. This mode involves expressing the cylinder number, head number, and sector number (CHS) by use of logical serial numbers called LBA starting from 0.
On conventional hard disc drives, the magnetic head starts scanning the media in an operation called a “seek” before reaching the track containing the target sector. When the track is reached, the magnetic head waits for the media to rotate during a period called a “rotational delay” until the target sector comes immediately under the magnetic head.
With the capacity of hard discs getting larger all the time, the track density is getting higher and the width of each track narrower. This means that the magnetic head must be positioned with high precision in order to write and read data precisely to and from sectors on the disc. The magnetic head is always positioned to the track center using the technique known as servo. The servo technique involves having “servo pattern” signals written beforehand to each track at fixed intervals. These signals are later read by the magnetic head so as to determine whether the magnetic head is accurately in the track center. In the stage of HDD fabrication, the servo patterns are written to the tracks on the hard disc in a highly accurate manner. Servo areas made up of the written servo patterns illustratively include a head-positioning signal, a cylinder number, a head number, and a servo number each.
Many conventional hard disc drives have interfaces such as IDE and SCSI for connecting with their host computer. The computer controls the disc drive using a command set defined by the interface in use. Basically, LBA designating a first sector and the number of sectors to be accessed therefrom are addressed. In turn, the hard disc drive gains access to the designated first sector. The HDD proceeds with its access operation by generating a look-ahead sequence for predicting the subsequent sectors to be accessed.
The look-ahead operation is based on the assumption that sectors having consecutive addresses are assigned to successive data. Generally, the sectors having consecutive addresses exist in conjunction with successive head numbers or track numbers. Where large quantities of data are written successively on the media, the look-ahead operation is effectively performed while the data are being read.
Where storage areas are so fragmented that large data are dispersed in small pieces over multiple locations, the look-ahead operation is ineffective because the operation points to discontinuous data. This phenomenon occurs partly because the hard disc drive has no grasp of the structure of files handled by the host (i.e., computer) that requests reading and writing of data.
If the prediction fails due to a new access request from the host, the disc drive seeks the track containing the sector where the requested data are located. With the tracking completed, the disc drive waits for the disc to turn until the target sector can be accessed. This is how seek time and rotational delay occur.
The look-ahead data are preserved as long as data buffer capacity is not exhausted. If the prediction fails continuously or intermittently, the old data in the data buffer are discarded in order of storage. Seek cannot be initiated while a look-ahead operation is being performed.
That is, there are time losses due to seek time, rotational delay, and seeks getting initiated by look-ahead operations which turn out to be ineffective, as well as data losses caused by the ineffective look-ahead operations.
In order to minimize seek time and rotational delay, conventional disc drives increase their disc revolutions. This measure is taken because of the difficulty in adopting an optimal access method since there is no regularity in the quantities and structures of data handled by the host (e.g., computers). However, raising the disc revolutions is disadvantageous in terms of power dissipation and storage capacity.
Many conventional external storage systems such as hard disc drives correct errors on a sector-by-sector basis (one sector is generally made up of 512 bytes). This feature is effective against random errors occurring within each sector but not against random errors exceeding the intra-sector correctable range or against burst errors. The latter types of errors have been dealt with conventionally by retry operations that keep read errors below a predetermined level.
Such retry operations, however, require waiting for the disc to make another turn for rereading the target data, which prolongs the data read time. For example, a system handling AV content may run into a situation requiring high-speed data transfers for high-definition image reproduction or special-effects playback. In such a case, uncorrectable errors within a sector cannot be retried for correction due to time constraints. Given the state of the art today, the reproducing process must be allowed to proceed without error correction. This has often resulted in reduced levels of image reproduction quality.
Japanese Patent Laid-open No. 2000-278645 discloses techniques for providing data blocks destined for recording with information indicative of their importance so that important data blocks will be retried for error correction if necessary while the other data blocks are excluded from retry operations. The techniques are shown to be used changeably between the two categories of data blocks.
Japanese Patent Laid-open No. 2000-276856 discloses techniques for providing data blocks destined for recording with information representative of their importance so that important data blocks will have an enhanced capability to be corrected for error while the other data blocks are given normal capability for error correction. The techniques are also used changeably between the two categories of data blocks.
The techniques above function more or less satisfactorily with systems handing AV content. However, there is a need for more effective techniques for averting retries and correcting errors.
Before being shipped from their factory, hard disc drives are assigned addresses so that certain sectors deemed faulty on the disc will not be accessed. The spots carrying these sectors are called slips. On any given track, several or dozens of designated sectors may be “slipped”. If a substantial number of sectors (e.g., dozens of sectors) are slipped per track, that translates into a considerable reduction in the effective per-track area of data sectors. This has a detrimental effect on data transfer rates.
If vibrations or other external disturbances take place while AV content is being retrieved from the medium, more errors tend to occur than if such disturbances were nonexistent. This means that there are more data which cannot be corrected for errors, so that reproduction quality is worsened.
In particular, if errors occur under external disturbances, more errors tend to occur intermittently in a sector segment flanked by servo areas located radially on the hard disc. These errors may be random errors or burst errors. In any case, the greater the magnitude of the external disturbance, the larger the number of sectors within which random errors cannot be corrected.